Correct and Appropriate Signal Transduction Processes Are Generally Under Strong

Multiple Choice

Correct and appropriate signal transduction processes are generally under strong selective pressure and are determined by the properties of the molecules involved, the concentrations of signal and receptor molecules, and the binding affinities between signal and receptor. Therefore, a hormone action is very specific in a species at any one point in time. However, there are examples of very diverse functions of a specific hormone between groups of organisms. For example, thyroxin, which is produced in all vertebrates and many invertebrates, can trigger growth, differentiation, metamorphosis, maturation, reproduction, behavior, temperature tolerance, osmoregulation, or seasonal adaptation depending on the organism in which it is produced. What is the most logical explanation for such different responses triggered by thyroxin in organisms?

A) The concentration of thyroxin varies in different organisms. Invertebrate organisms do not have as much thyroxin as vertebrate organisms.
B) Thyroxin and its receptor molecules have a different binding affinity in different organisms.
C) Receptor molecules for thyroxin are located on different tissues in different organisms.
D) The function of thyroxine in a particular tissue is determined by the genes that are transcribed by the activated thyroxine receptor.